Support for extending native classes

sdk/lib/_internal/compiler/implementation/js_backend/namer.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart b/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
index 796718a952e1332d816ccf878f07482f20233575..94fb10838a992dcac9f4fc0d0ea78fbd308f4053 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
@@ -566,11 +566,11 @@ class Namer implements ClosureNamer {
       return cls.name.slowToString();
     }
     List<String> names = classes
-        .where((cls) => !cls.isNative())
+        .where((cls) => !Elements.isNativeOrExtendsNative(cls))
         .map(abbreviate)
         .toList();
     // There is one dispatch mechanism for all native classes.
-    if (classes.any((cls) => cls.isNative())) {
+    if (classes.any((cls) => Elements.isNativeOrExtendsNative(cls))) {
       names.add("x");
     }
     // Sort the names of the classes after abbreviating them to ensure